The output powers, efficiencies, and lifetimes of short-wavelength ultraviolet light-emitting diodes (UV LEDs)—i.e., LEDs that emit light at wavelengths less than 350 nm—based on the nitride semiconductor system remain limited due to high defect levels in the active region. These limitations are particularly problematic (and notable) in devices designed to emit at wavelengths less than 280 nm. Particularly in the case of devices formed on foreign substrates, such as sapphire, defect densities remain high despite significant efforts to reduce them. These high defect densities limit both the efficiency and the reliability of devices grown on such substrates.
The recent introduction of low-defect, crystalline aluminum nitride (AlN) substrates has the potential to dramatically improve nitride-based optoelectronic semiconductor devices, particularly those having high aluminum concentration, due to the benefits of having lower defects in the active regions of these devices. For example, UV LEDs pseudomorphically grown on AlN substrates have been demonstrated to have higher efficiencies, higher power, and longer lifetimes compared to similar devices formed on other substrates. Generally, these pseudomorphic UV LEDs are mounted for packaging in a “flip-chip” configuration, where the light generated in the active region of the device is emitted through the AlN substrate, while the LED dies have their front surfaces (i.e., the top surfaces of the devices during epitaxial growth and initial device fabrication prior to bonding) bonded to a patterned submount which is used to make electrical and thermal contact to the LED chip. A good submount material is polycrystalline (ceramic) AlN because of the relatively good thermal expansion match with the AlN chip and because of the high thermal conductivity of this material. Due to the high crystalline perfection that is achievable in the active device region of such devices, internal efficiencies greater than 60% have been demonstrated.
Unfortunately, the photon-extraction efficiency is often still very poor in these devices, ranging from about 4% to about 15% achieved using surface-patterning techniques—much lower than exhibited by many visible-light (or “visible”) LEDs. Thus, the current generation of short-wavelength UV LEDs has low wall-plug efficiencies (WPEs) of, at best, only a few percent, where the WPE is defined as the ratio of usable optical power (in this case, emitted UV light) achieved from the diode to the electrical power supplied into the device. The WPE of an LED may be calculated by taking the product of the electrical efficiency (ηel), the photon extraction efficiency (ηex), and the internal efficiency (IE); i.e., WPE=ηel×ηex×IE. The IE itself is the product of current injection efficiency (ηinj) and the internal quantum efficiency (IQE); i.e., IE=ηinj×IQE. Thus, a low ηex will deleteriously impact the WPE even after the IE has been improved via the reduction of internal crystalline defects enabled by, e.g., the use of the AlN substrates referenced above as platforms for the devices.
There are several possible contributors to low photon-extraction efficiency. For example, currently available AlN substrates generally have some absorption in the UV wavelength range, even at wavelengths longer than the band edge in AlN (which is approximately 210 nm). This absorption tends to result in some of the UV light generated in the active area of the device being absorbed in the substrate, hence diminishing the amount of light emitted from the substrate surface. However, this loss mechanism may be mitigated by thinning the AlN as described in U.S. Pat. No. 8,080,833 (“the '833 patent,” the entire disclosure of which is incorporated by reference herein) and/or by reducing the absorption in the AlN substrate as described in U.S. Pat. No. 8,012,257 (the entire disclosure of which is incorporated by reference herein). Additionally, UV LEDs typically suffer because approximately 50% of the generated photons are directed toward the p-contact, which typically includes photon-absorbing p-GaN. Even when photons are directed toward the AlN surface, only about 9.4% typically escape from an untreated surface due to the large index of refraction of the AlN, which results in a small escape cone. These losses are multiplicative and the average photon extraction efficiency may be quite low.
As demonstrated in a recent publication by Grandusky et al. (James R. Grandusky et al., 2013 Appl. Phys. Express, Vol. 6, No. 3, 032101, hereinafter referred to as “Grandusky 2013,” the entire disclosure of which is incorporated by reference herein), it is possible to increase the photon extraction efficiency to approximately 15% in pseudomorphic UV LEDs grown on AlN substrates via the attachment of an inorganic (and typically rigid) lens directly to the LED die via a thin layer of an encapsulant (e.g., an organic, UV-resistant encapsulant compound). This encapsulation approach, which is also detailed in U.S. patent application Ser. No. 13/553,093, filed on Jul. 19, 2012 (“the '093 application,” the entire disclosure of which is incorporated by reference herein), increases the critical angle of total internal reflection through the top surface of the semiconductor die, which significantly improves photon-extraction efficiency for the UV LEDs. In addition, and as mentioned above, the photon extraction efficiency may be increased by thinning the AlN substrate and by roughening the surface of the AlN substrate surface as discussed in the '833 patent.
Unfortunately, none of these efforts addresses the major loss of photons due to absorption in the p-GaN utilized for the p-contact to these devices. In the type of pseudomorphic UV device described by Grandusky 2013, p-GaN is used to make the p-contact to the LED because it allows a relatively low resistance contact to be made to the p-side of the device. However, the band gap energy of GaN is only 3.4 eV, and thus it is highly absorbing to photons with wavelengths shorter than 365 nm. Since typically 50% of the photons generated are directed toward the p-contact, these photons are typically immediately lost due to absorption in the p-GaN. In addition, even photons directed toward the emission surface of the diode will typically only have a single chance to escape since, if they are reflected back into the diode, they will likely be absorbed by the p-GaN. The p-GaN is utilized conventionally because it is very difficult to make a low-resistivity contact to p-AlxGa1-xN where x is greater than 0.3. In addition, metals that allow low-resistivity contact to the p-type nitride semiconductor material are generally poor reflectors. This reflectivity problem is particularly exacerbated when the desired wavelength of the LED is less than 340 nm since most common metals will start to absorb strongly in that regime.
In addition, prior work has suggested using a thick p-GaN layer (or p-AlxGa1-xN layer with x<0.2) so that the hole current spreads sufficiently from and beneath the p-metal contacts. This approach generally will not work for devices emitting light of wavelengths shorter than 300 nm because of the high absorption of the p-GaN or p-AlxGa1-xN material at these shorter wavelengths.
Alternatively, the above-referenced shortcomings might be remedied via the use of a non-absorbing p-type semiconductor on the p-side of the LED and the use of p-contact metallurgy that reflects the UV photons. However, conventional approaches are unsuited to pseudomorphic UV LEDs since these approaches use multiple layers of thin p-AlxGa1-xN where the p-type AlxGa1-xN layers are thin enough to be optically transparent to the UV radiation at wavelengths shorter than 300 nm. This type of multi-layer structure is very difficult to grow on a pseudomorphic device structure (where the underlying substrate is either AlN or AlxGa1-xN with x>0.6), because the large amount of strain (due to the lattice mismatch) typically causes the thin GaN (or low aluminum content AlxGa1-xN) to island and become very rough. In the Grandusky 2013 paper, contact roughening is addressed by making the p-type GaN layer quite thick; however, such layers, as detailed above, absorb UV photons and diminish UV LED device efficiencies.
Therefore, in view of the foregoing, there is a need for improved contact metallurgy and performance for UV LEDs, particularly those UV LEDs produced on AlN substrates, in order to improve characteristics, such as the WPE, of such devices.